[PATCH] x86: correct default_xen_spec_ctrl calculation

[PATCH] x86: correct default_xen_spec_ctrl calculation

[Jan Beulich]

Even with opt_msr_sc_{pv,hvm} both false we should set up the variable
as usual, to ensure proper one-time setup during boot and CPU bringup.
This then also brings the code in line with the comment immediately
ahead of the printk() being modified saying "irrespective of guests".

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -195,8 +195,6 @@ custom_param("spec-ctrl", parse_spec_ctr
 
 static void __init print_details(enum ind_thunk thunk, uint64_t caps)
 {
-    bool use_spec_ctrl = (boot_cpu_has(X86_FEATURE_SC_MSR_PV) ||
-                          boot_cpu_has(X86_FEATURE_SC_MSR_HVM));
     unsigned int _7d0 = 0, e8b = 0, tmp;
 
     /* Collect diagnostics about available mitigations. */
@@ -228,10 +226,9 @@ static void __init print_details(enum in
            thunk == THUNK_RETPOLINE ? "RETPOLINE" :
            thunk == THUNK_LFENCE    ? "LFENCE" :
            thunk == THUNK_JMP       ? "JMP" : "?",
-           !use_spec_ctrl                            ?  "No" :
-           (default_xen_spec_ctrl & SPEC_CTRL_IBRS)  ?  "IBRS+" :  "IBRS-",
-           !use_spec_ctrl || !boot_cpu_has(X86_FEATURE_SSBD)
-                                                     ? "" :
+           !boot_cpu_has(X86_FEATURE_IBRSB)          ? "No" :
+           (default_xen_spec_ctrl & SPEC_CTRL_IBRS)  ? "IBRS+" :  "IBRS-",
+           !boot_cpu_has(X86_FEATURE_SSBD)           ? "" :
            (default_xen_spec_ctrl & SPEC_CTRL_SSBD)  ? " SSBD+" : " SSBD-",
            opt_ibpb                                  ? " IBPB"  : "");
 
@@ -493,16 +490,14 @@ void __init init_speculation_mitigations
         }
 
         if ( use_spec_ctrl )
-        {
-            if ( ibrs )
-                default_xen_spec_ctrl |= SPEC_CTRL_IBRS;
-
             default_spec_ctrl_flags |= SCF_ist_wrmsr;
-        }
+
+        if ( ibrs )
+            default_xen_spec_ctrl |= SPEC_CTRL_IBRS;
     }
 
     /* If we have SSBD available, see whether we should use it. */
-    if ( boot_cpu_has(X86_FEATURE_SSBD) && use_spec_ctrl && opt_ssbd )
+    if ( boot_cpu_has(X86_FEATURE_SSBD) && opt_ssbd )
         default_xen_spec_ctrl |= SPEC_CTRL_SSBD;
 
     /*





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Re: [PATCH] x86: correct default_xen_spec_ctrl calculation

[Andrew Cooper]

On 25/05/18 12:08, Jan Beulich wrote:
> Even with opt_msr_sc_{pv,hvm} both false we should set up the variable
> as usual, to ensure proper one-time setup during boot and CPU bringup.
> This then also brings the code in line with the comment immediately
> ahead of the printk() being modified saying "irrespective of guests".
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Hmm yes - I think the logic here got a bit tangled over the course of
development.

AFAICT, this should work, so tentatively Reviewed-by: Andrew Cooper
<andrew.cooper3@citrix.com>

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Re: [PATCH] x86: correct default_xen_spec_ctrl calculation

[Juergen Gross]

On 25/05/18 13:08, Jan Beulich wrote:
> Even with opt_msr_sc_{pv,hvm} both false we should set up the variable
> as usual, to ensure proper one-time setup during boot and CPU bringup.
> This then also brings the code in line with the comment immediately
> ahead of the printk() being modified saying "irrespective of guests".
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Release-acked-by: Juergen Gross <jgross@suse.com>


Juergen

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